Bad radiator cap or a bad head gasket? Here’s how to know

You don’t know if it’s a bad radiator cap or a bad head gasket? No problem, I’ll show you how to fix that. Here’s how:

Look out for common signs of a bad radiator cap and signs of a bad head gasket. A faulty radiator cap will result in an overheating engine and a leaking coolant. You may also have an overflowing coolant reservoir tank and a collapsed radiator hose.

A bad gasket, on the other hand, will similarly cause an overheating engine, and coolant loss with no leaks. You may also see some bubbling in the radiator cap and coolant reservoir, not to mention, white smoke coming from the tailpipe.

How to Know If You Have a Bad Radiator Cap

  • An overheating engine.
  • A leaking coolant.
  • An overflowing coolant reservoir tank.
  • A collapsed radiator hose.

Let’s look at them one by one

An Overheating Engine

The job of the radiator cap is to let out excess pressure from the coolant, ensuring the coolant doesn’t overheat too early(i.e when the engine is still at normal temperatures, between 75°C and 105°C).

If it becomes bad, the coolant will overheat due to excessive pressure and so will the engine.

A Leaking Coolant

Your radiator cap can be bad or faulty in many ways including having a seal that doesn’t seal properly. Both the upper seal and the lower seal that is made of rubber should seal the cap properly failure to which the coolant will leak.

An Overflowing Coolant Reservoir Tank

The coolant reservoir tank’s job is to store excess coolant for later use. You see when the coolant is heated up and pressurized it expands such that some of it goes into the tank.

If the radiator cap is bad, the coolant will be overly pressurized and overheated. Consequently, it’ll expand too much. As such, coolant exceeding the tank’s capacity will flow into the tank such that some of it will overflow.

A Collapsed Radiator Hose

Again, if the radiator cap isn’t working, the radiator will become overheated and overpressurized. This will cause the attached hose to collapse since rubber is generally too delicate to withstand this amount of heat and pressure.

How to Know If You Have a Bad Head Gasket

To start with, the head gasket’s role is to seal the engine’s combustion chamber. This allows the engine to build adequate pressure necessary to maintain the power.

This component also keeps the coolant from leaking, thus preventing the engine from overheating.

As said earlier, common symptoms of a faulty head gasket are as follows:

  • Overheating engine
  • Coolant loss with no leaks or any other explanation.
  • Bubbling in the radiator and coolant reservoir.
  • White smoke coming from the tailpipe
  • Some milky white coloration in the oil.

Let’s inspect each symptom:

Overheating Engine

If your head gasket is faulty, it may not be able to prevent your coolant from leaking, by sealing the coolant paths effectively. That means there won’t be sufficient coolant to cool the engine adequately. Meaning your engine will overheat.

Coolant Loss With No Leaks

As you already know, a bad head gasket leads to the overheating of the engine. This causes the boiling of the coolant where some fluid turns to vapor and evaporates away. As a result, you’ll notice coolant loss despite the absence of leakage.

Bubbling in the Radiator Cap and Coolant Reservoir

A bad radiator cap or head gasket will cause your coolant to overheat quickly and boil. Then it will expand into both the cap and reservoir.

White Smoke Coming from the Exhaust

Besides expanding into the radiator cap and coolant reservoir, the boiling coolant may also find its way into the combustion cylinders and get burned together with the fuel. This will result in white smoke that will come out of the car through the tailpipe.

Solutions for a Bad Radiator Cap (Including Replacement/Repair costs)

More often than not, a bad radiator cap is simply replaced. The good news is that this is usually very affordable with the average replacement cost ranging between $10 and $50, depending on the car model and garage. Labor is often free as it takes a matter of seconds.

Solutions for a Bad Head Gasket (with replacement/repair costs)

Similarly, a bad head gasket is often replaced rather than repaired. According to a national average, the average cost of head gasket repair is around $1624 to $1979 inclusive of labor.

Labor is the most expensive fraction of the cost as it’s extremely time-consuming since the engine head has to be removed and then replaced.


Modern cars come with a pressurized cooling system that consists of the radiator cap and the head gasket among other things.

Pressurizing helps raise the coolant’s boiling point, preventing it from overheating and boiling prematurely(i.e when the engine is still at normal temperatures: 75°C-105°C).

Unfortunately, excess pressure can destroy the radiator and the components attached to it including the hose.

To prevent that, the radiator is equipped with a cap that automatically opens and releases excess pressure. The head gasket which is found on the engine block also facilitates its work by sealing the engine’s combustion chamber so that it can build this vital coolant pressure.

It also prevents the coolant from leaking and causing many problems that can otherwise affect the radiator cap’s effectiveness.

If you have a bad radiator cap or head gasket, replacing the part is the most viable solution in either case.

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